Why is urochordates and cephalochordates called Protochordates?

Explanation

### Step-by-Step Solution

1. **Definition of Protochordates**: Urochordates and cephalochordates are referred to as protochordates because they represent a primitive form of chordates. The term "proto" means "first" or "primitive," indicating that these organisms exhibit early characteristics of chordates.

2. **Subphylum Classification**: Both urochordates and cephalochordates belong to a separate subphylum within the phylum Chordata. They are not classified as fully developed chordates, which is why they are grouped separately.

3. **Lack of Advanced Features**: Protochordates are considered primitive because they lack certain advanced features that are characteristic of higher chordates. For instance, they do not possess a fully developed brain.

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